1. Field of the Invention
The present invention relates generally to molded composite materials. More particularly, the present invention is directed to molded composite materials that are suitable for structural applications and which can be machined accurately to provide precise fitting of parts together or other benefits.
2. Description of Related Art
Composite materials typically include fibers and a resin matrix as the two principal components. Composite materials typically have a rather high strength to weight ratio. As a result, composite materials are being used in demanding environments, such as in the field of aerospace where the high strength and relatively light weight of composite parts are of particular importance.
Compression molding of thermosetting or thermoplastic composite materials is often used to produce parts that connect major structures together. For example, molded composite materials have been used as fittings, gussets, clips, cleats and intercostals that connect major structural parts of an airplane together. The molded composite connector must mate precisely with the various structures, such as fuselage parts, wing skins, frames, stiffeners and spars, which are being connected together.
Metal parts have been used in the past to connect aircraft structures together. An advantage of using metal connectors is that they can be machined to precisely match the geometry of the mating parts as well as meet the tolerances required for proper assembly. In many instances, the same structural assembly is repeated many times, but each instance has a unique geometry, because of the aerodynamic shapes of wings and fuselages. Fuselage and wing cross-sections are not constant. For example, wings taper from the wing root (near the fuselage) to the wing tip. Fuselage cross-sections are reduced toward the nose and tail of the airplane. The skin stiffening structures for the fuselage and wing are made of an assembly of regularly spaced circumferential frames and regularly spaced longitudinal stringers. The geometry of the connectors that are used to tie the frames and stringers to the skin vary slightly from one location to the next due to the non-constant cross-section of the wing or fuselage skins. It is an expensive proposition to make a different mold for each particular connector at a given location. It would be desirable to mold a single part that could then be machined to provide a number of different parts that have the required variations in part geometry. A single mold could be used to produce multiple parts with slightly different geometries, for example to accommodate the slight change of geometry of a fuselage skin from one frame location to the next, instead of the current practice which is limited to producing a single part geometry per mold.
It is very difficult to produce molded composite parts that have tolerances which are as tight as the tolerances that can be obtained with a machined metal part. In addition, a molded composite part cannot be remolded to meet the changes in dimensions that occur due to variables in the assembly process. As a result, shims are typically used to correct the dimensional mismatches that may occur when molded composite parts are used to connect structures together.
It is difficult to machine composite parts without creating structural damage. The fibers tend to be disrupted and delamination can occur which greatly reduces the strength of the part. Composite parts that contain unidirectional (UD) fibers are commonly used in many structural parts. Composite parts that contain UD fibers are particularly difficult to machine without adversely affecting the strength of the part. Accordingly, machining of such structural composite parts has been avoided
It would be desirable to provide composite parts that can be used in structural applications and which can be manufactured to precise initial dimensions. In addition, the dimensions of the structural composite part should be amenable to alteration at a later time, if needed, to ensure that the part fits properly without the need for shimming.